Constify struct flash_driver instances

Instances of struct flash_driver are never written to at runtime. For a
small amount of memory saving and also robustness (fewer things for
stray pointer writes to hit), mark them const.

Change-Id: Iadbbbc2fac0976d892699200000c5f02856729f3
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4803
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/flash/nor/numicro.c

@@ -1871,7 +1871,7 @@ static const struct command_registration numicro_command_handlers[] = {
 	COMMAND_REGISTRATION_DONE
 };
 
-struct flash_driver numicro_flash = {
+const struct flash_driver numicro_flash = {
 	.name = "numicro",
 	.commands = numicro_command_handlers,
 	.flash_bank_command = numicro_flash_bank_command,